This article makes two important contributions to the literature on the incentive effects of insider ownership. First, it presents a clean method for separating the positive wealth effect of insider ownership from the negative entrenchment effect, which can be applied to samples of companies from the US and any other country. Second, it measures the effects of insider ownership using a measure of firm performance, namely a marginal q, which ensures that the causal relationship estimated runs from ownership to performance. The article applies this method to a large sample of publicly listed firms from the Anglo-Saxon and Civil law traditions and confirms that managerial entrenchment has an unambiguous negative effect on firm performance as measured by both Tobin's (average) q and our marginal q, and that the wealth effect of insider ownership is unambiguously positive for both measures. We also test for the effects of ownership concentration for other categories of owners and find that while institutional ownership improves the performance in the USA, financial institutions have a negative impact in other Anglo-Saxon countries and in Europe.